Catecholamines and the sodium pump in excitable cells

The importance of systemic response in the pathobiology of blast-induced neurotrauma

Due to complex injurious environment where multiple blast effects interact with the body parallel, blast-induced neurotrauma is a unique clinical entity induced by systemic, local, and cerebral responses. Activation of autonomous nervous system; sudden pressure increase in vital organs such as lungs and liver; and activation of neuroendocrine-immune system are among the most important mechanisms that contribute significantly to molecular changes and cascading injury mechanisms in the brain. It has been hypothesized that vagally mediated cerebral effects play a vital role in the early response to blast: this assumption has been supported by experiments where bilateral vagotomy mitigated bradycardia, hypotension, and apnea, and also prevented excessive metabolic alterations in the brain of animals exposed to blast. Clinical experience suggests specific blast-body-nervous system interactions such as (1) direct interaction with the head either through direct passage of the blast wave through the skull or by causing acceleration and/or rotation of the head; and (2) via hydraulic interaction, when the blast overpressure compresses the abdomen and chest, and transfers its kinetic energy to the body's fluid phase, initiating oscillating waves that traverse the body and reach the brain. Accumulating evidence suggests that inflammation plays important role in the pathogenesis of long-term neurological deficits due to blast. These include memory decline, motor function and balance impairments, and behavioral alterations, among others. Experiments using rigid body- or head protection in animals subjected to blast showed that head protection failed to prevent inflammation in the brain or reduce neurological deficits, whereas body protection was successful in alleviating the blast-induced functional and morphological impairments in the brain.

Differential effects of Na+-K+ ATPase blockade on cortical layer V neurons

Sodium-potassium ATPase ('Na(+)-K(+) ATPase') contributes to the maintenance of the resting membrane potential and the transmembrane gradients for Na(+) and K(+) in neurons. Activation of Na(+)-K(+) ATPase may be important in controlling increases in intracellular sodium during periods of increased neuronal activity. Down-regulation of Na(+)-K(+) ATPase activity is implicated in numerous CNS disorders, including epilepsy. Although Na(+)-K(+) ATPase is present in all neurons, little is known about its activity in different subclasses of neocortical cells. We assessed the physiological properties of Na(+)-K(+) ATPase in fast-spiking (FS) interneurons and pyramidal (PYR) cells to test the hypothesis that Na(+)-K(+) ATPase activity would be relatively greater in neurons that generated high frequency action potentials (the FS cells). Whole-cell patch clamp recordings were made from FS and PYR neurons in layer V of rat sensorimotor cortical slices maintained in vitro using standard techniques. Bath perfusion of Na(+)-K(+) ATPase antagonists (ouabain or dihydro-ouabain) induced either a membrane depolarization in current clamp, or inward current under voltage clamp in both cell types. PYR neurons were divided into two subpopulations based on the amplitude of the voltage or current shift in response to Na(+)-K(+) ATPase blockade. The two PYR cell groups did not differ significantly in electrophysiological properties including resting membrane potential, firing pattern, input resistance and capacitance. Membrane voltage responses of FS cells to Na(+)-K(+) ATPase blockade were intermediate between the two PYR cell groups (P < 0.05). The resting Na(+)-K(+) ATPase current density in FS interneurons, assessed by application of blockers, was 3- to 7-fold larger than in either group of PYR neurons. Na(+)-K(+) ATPase activity was increased either through direct Na(+) loading via the patch pipette or by focal application of glutamate (20 mM puffs). Under these conditions FS interneurons exhibited the largest increase in Na(+)-K(+) ATPase activity. We conclude that resting Na(+)-K(+) ATPase activity and sensitivity to changes in internal Na(+) concentration vary between and within classes of cortical neurons. These differences may have important consequences in pathophysiological disorders associated with down-regulation of Na(+)-K(+) ATPase and hyperexcitability within cortical networks.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

Concentration-dependent effects of anticonvulsant enaminone methyl 4-(4'-bromophenyl)aminocyclohex-3-en-6-methyl-2-oxo-1-oate on neuronal excitability in vitro

Enaminones are a novel group of compounds some of which possess anticonvulsant activity in in vivo animal models of seizures. We recently reported that some enaminones, including methyl 4-(4'-bromophenyl)aminocyclohex-3-en-6-methyl-2-oxo-1-oate, depress glutamate-mediated excitatory synaptic transmission and that this may contribute to their anticonvulsant activity [Kombian SB, Edafiogho IO, Ananthalakshmi KVV (2005) Anticonvulsant enaminones depress excitatory synaptic transmission in the rat brain by enhancing extracellular GABA levels. Br J Pharmacol 145:945-953]. Here we studied the effects of methyl 4-(4'-bromophenyl)aminocyclohex-3-en-6-methyl-2-oxo-1-oate, on the excitability of male rat (Sprague-Dawley) nucleus accumbens and hippocampal cells in vitro using whole-cell patch clamp recording techniques. At low, therapeutically relevant concentrations (0.3-10 microM), methyl 4-(4'-bromophenyl)aminocyclohex-3-en-6-methyl-2-oxo-1-oate reversibly suppressed action potential firing rate in a concentration-dependent manner. This action potential suppression was present when GABA(A), GABA(B) and glutamate receptors were blocked with their antagonists. Furthermore, methyl 4-(4'-bromophenyl)aminocyclohex-3-en-6-methyl-2-oxo-1-oate suppressed tetrodotoxin-sensitive sodium currents in these cells. At concentrations >/=100 microM, it induced inward currents and increased action potential firing frequency. The inward currents were without changes in input resistance and did not reverse polarity between -120 and -40 mV. These currents were independent of extracellular potassium, but were absent when extracellular sodium was replaced by choline and finally, were occluded by pretreatment with ouabain (200 microM). We conclude that methyl 4-(4'-bromophenyl)aminocyclohex-3-en-6-methyl-2-oxo-1-oate directly inhibits action potential firing at therapeutically relevant concentrations by suppressing tetrodotoxin-sensitive sodium currents, while inducing an ouabain-sensitive current at high concentrations to excite neurons. These two actions of methyl 4-(4'-bromophenyl)aminocyclohex-3-en-6-methyl-2-oxo-1-oate on neuronal excitability would have therapeutic implications in future clinical use of enaminones as anticonvulsants in seizure disorders.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Regulation of glial Na+/K+-ATPase by serotonin: identification of participating receptors

The purpose of the present study was the characterization of the receptors participating in the regulatory mechanism of glial Na+/K+-ATPase by serotonin (5-HT) in rat brain. The activity of the Na+ pump was measured in four brain regions after incubation with various concentrations of serotoninergic agonists or antagonists. A concentration-dependent increase in enzyme activity was observed with the 5-HT1A agonist R (+)-2-dipropylamino-8-hydroxy-1,2,3, 4-tetrahydronaphthalene hydrobromide (8-OH-DPAT) in homogenates or in glial membrane enriched fractions from cerebral cortex and in hippocampus. Spiperone, a 5-HT1A antagonist, completely inhibited the response to 8-OH-DPAT but had no effect on Na+/K+-ATPase activity in cerebellum where LSD, a 5-HT6 agonist, elicited a dose-dependent response similar to that of 5-HT. In brainstem, a lack of response to 5-HT and other agonists was confirmed. Altogether, these results show that serotonin modulates glial Na+/K+-ATPase activity in the brain, apparently not through only one type of 5-HT receptor. It seems that the receptor system involved is different according to the brain region. In cerebral cortex, the response seems to be mediated by 5-HT1A as well as in hippocampus but not in cerebellum where 5-HT6 appears as the receptor system involved.

Inhibitory action of CGS 21680 on cerebral cortical neurons is antagonized by bicuculline and picrotoxin-is GABA involved?

The possibility of an involvement of endogenously released GABA in the inhibitory actions of A1 and A2a adenosine receptor agonists on rat cerebral cortical neurons discharges was examined using the GABAA antagonists bicuculline and picrotoxin. The A1 agonist N6-cyclopentyladenosine (CPA), the A2a agonist CGS 21680 and the non-selective receptor agonist, adenosine, depressed neuronal firing. Applications of bicuculline or picrotoxin enhanced the spontaneous firing rate of cortical neurons, indicating the presence of ongoing GABA-ergic inhibition. Antagonism of GABAA receptors blocked the depressant effects of CGS 21680 on neuronal firing; was without effect on CPA-evoked inhibitions and tended to reduce the duration of adenosine-evoked inhibitions. These results suggest that the depressant effects of A2a receptor activation are due to an increase in GABA-ergic inhibition, likely as a consequence of increased GABA release. GABA does not appear to be involved in adenosine A1 receptor-mediated inhibition of neuronal firing.

Ionic mechanisms of muscarinic depolarization in entorhinal cortex layer II neurons

The mechanisms underlying direct muscarinic depolarizing responses in the stellate cells (SCs) and non-SCs of medial entorhinal cortex layer II were investigated in tissue slices by intracellular recording and pressure-pulse applications of carbachol (CCh). Subthreshold CCh depolarizations were largely potentiated in amplitude and duration when paired with a short DC depolarization that triggered cell firing. During Na+ conductance block, CCh depolarizations were also potentiated by a brief DC depolarization that allowed Ca2+ influx and the potentiation was more robust in non-SCs than in SCs. Also, in non-SCs, CCh depolarizations could be accompanied by spikelike voltage oscillations at a slow frequency. In both SCs and non-SCs, the voltage-current (V-I) relations were similarly affected by CCh, which caused a shift to the left of the steady-state V-I relations over the entire voltage range and an increase in apparent slope input resistance at potentials positive to about -70 mV. CCh responses potentiated by Ca2+ influx demonstrated a selective increase in slope input resistance at potentials positive to about -75 mV in relation to the nonpotentiated responses. K+ conductance block with intracellular injection of Cs+ (3 M) and extracellular Ba2+ (1 mM) neither abolished CCh depolarizations nor resulted in any qualitatively distinct effect of CCh on the V-I relations. CCh depolarizations were also undiminished by block of the time-dependent inward rectifier Ih, with extracellular Cs . However, CCh depolarizations were abolished during Ca2+ conductance block with low-Ca2+ (0.5 mM) solutions containing Cd2+, Co2+, or Mn2+, as well as by intracellular Ca2+ chelation with bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid. Inhibition of the Na+-K+ ATPase with strophanthidin resulted in larger CCh depolarizations. On the other hand, when NaCl was replaced by N-methyl-D-glucamine, CCh depolarizations were largely diminished. CCh responses were blocked by 0.8 microM pirenzepine, whereas hexahydro-sila-difenidolhydrochloride,p-fluoroanalog (p-F-HHSiD) and himbacine were only effective antagonists at 5- to 10-fold larger concentrations. Our data are consistent with CCh depolarizations being mediated in both SCs and non-SCs by m1 receptor activation of a Ca2+-dependent cationic conductance largely permeable to Na+. Activation of this conductance is potentiated in a voltage-dependent manner by activity triggering Ca2+ influx. This property implements a Hebbian-like mechanism whereby muscarinic receptor activation may only be translated into substantial membrane depolarization if coupled to postsynaptic cell activity. Such a mechanism could be highly significant in light of the role of the entorhinal cortex in learning and memory as well as in pathologies such as temporal lobe epilepsy.

Activation of the Na(+)-K+ pump in frog erythrocytes by catecholamines and phosphodiesterase blockers

K+ and Na+ influx into frog erythrocytes incubated in standard saline was studied using 86Rb and 22Na as tracers. 10 microM isoproterenol (ISP) produced a significant increase in K+ influx for the first 15 min, which was sustained during the entire 60 min of cell incubation. Treatment of red cells with the phosphodiesterase (PDE) blockers theophylline (THEO, 1 and 5 mM) or 3-isobutyl-1-methylxanthine (IBMX, 0.5 mM) was also accompanied by an enhancement in K+ influx. A distinct additive effect on K+ influx into red cells was found when ISP and THEO or IBMX were added together. The increase in K+ transport induced by ISP plus IBMX was totally abolished by pretreatment of red cells with 0.1 mM ouabain. The ouabain-sensitive K+ influx in frog erythrocytes was elevated in the presence of ISP plus IBMX to 2.05 +/- 0.45, as compared with the control level of 0.39 +/- 0.11 mmol/L cells/hr (P < 0.001). Similar effects of ISP and IBMX on K+ influx were observed after chloride was replaced by nitrate. A dose-related increase in K+ influx into frog erythrocytes was observed at ISP concentrations of 10(-8)-10(-6) M, with a half-maximal stimulatory concentration of approximately 0.02 microM. The effects of ISP (10(-8)-10(-5) M) on K+ transport were completely abolished with 10 microM of the beta-adrenergic blocker propranolol, but alpha-adrenergic antagonists (phentolamine, prazosin, and yohimbine) did not alter the ISP-induced increase in K+ influx. The drugs tested had no effect on 22Na influx in frog red cells, but ISP produced a small decline (13%) in intracellular Na+ concentration. Thus, our study indicates that catecholamines and PDE blockers enhance K+ (86Rb) transport in frog erythrocytes mediated by Na(+)-K+ pump activity. The frog erythrocyte membrane may serve as a convenient model to investigate the hormonal modulation of the Na(+)-K+.

Acute and chronic effects of potassium and noradrenaline on Na+, K+-ATPase activity in cultured mouse neurons and astrocytes

Acute and chronic effects of elevated extracellular concentrations of potassium ions ([K+]0) and/or noradrenaline were studied in homogenates of primary cultures of mouse astrocytes, from the cerebral cortex or the spinal cord, and of primary cultures of mouse cerebral cortical neurons. NA+, K+-ATPase activity in cerebral cortical astrocytes showed a Km value of 1.9 mM with confidence limits of 1.3-2.9 mM and a Vmax of 5.4 mumol/h/mg protein with confidence limits of 3.3-8.1 mumol/h/mg protein. Due to the high Km value, the activity of the enzyme was significantly increased by an increase in [K+]0 in the interval 5-12 mM. In cerebral cortical neurons, Vmax was lower (1.77 +/- 0.06 mumol/h/mg protein) but the affinity was higher (Km 0.43 +/- 0.8 mM). With these kinetics, there is no stimulation of enzyme activity when [K+]0 is increased beyond control levels. In spinal cord astrocytes, the relative effect of increasing [K+]0 above 6 mM was larger than in cerebral astrocytes but the absolute activity of the enzyme was lower. Na+, K+-ATPase activity in both types of astrocyte was stimulated by noradrenaline and its beta-adrenergic subtype agonist isoproterenol but mainly or exclusively at 6 mM [K+]0. Noradrenaline also caused a stimulation in cortical neurons, but at non-physiological K+ concentrations this stimulation was converted to an inhibition, and isoproterenol had no stimulatory effect. Chronic exposure of cerebral cortical astrocytes to elevated [K+]0 caused a decrease in Na+, K+-ATPase activity when enzyme activity in the cells was subsequently measured at normal [K+]0. During exposure to 30 mM [K+]0 this "down-regulation" took place within 10 min. Conversely, chronic exposure to reduced [K+]0 led to an increase in Na+, K+-ATPase activity. Chronic exposure to noradrenaline had no significant effect but there was a tendency towards an increase.

Vulnerability of medium spiny striatal neurons to glutamate: role of Na+/K+ ATPase

In Huntington's disease neuronal degeneration mainly involves medium-sized spiny neurons. It has been postulated that both excitotoxic mechanisms and energy metabolism failure are implicated in the neuronal degeneration observed in Huntington's disease. In central neurons, > 40% of the energy released by respiration is used by Na+/K+ ATPase to maintain ionic gradients. Considering that impairment of Na+/K+ ATPase activity might alter postsynaptic responsivity to excitatory amino acids (EAAs), we investigated the effects of the Na+/K+ ATPase inhibitors, ouabain and strophanthidin, on the responses to different agonists of EAA receptors in identified medium-sized spiny neurons electrophysiologically recorded in the current- and voltage-clamp modes. In most of the cells both ouabain and strophanthidin (1-3 microM) did not cause significant change in the membrane properties of the recorded neurons. Higher doses of either ouabain (30 microM) or strophanthidin (30 microM) induced, per se, an irreversible inward current coupled to an increase in conductance, leading to cell deterioration. Moreover, both ouabain (1-10 microM) and strophanthidin (1-10 microM) dramatically increased the membrane depolarization and the inward current produced by subcritical concentrations of glutamate, AMPA and NMDA. These concentrations of Na+/K+ ATPase inhibitors also increased the membrane responses induced by repetitive cortical activation. In addition, since it had previously been proposed that dopamine mimics the effects of Na+/K+ ATPase inhibitors and that dopamine agonists differentially regulate the postsynaptic responses to EAAs, we tested the possible modulation of EAA-induced membrane depolarization and inward current by dopamine agonists. Neither dopamine nor selective dopamine agonists or antagonists affected the postsynaptic responses to EAAs. Our experiments show that impairment of the activity of Na+/K+ ATPase may render striatal neurons more sensitive to the action of glutamate, lowering the threshold for the excitotoxic events. Our data support neither the role of dopamine as an ouabain-like agent nor the differential modulatory action of dopamine receptors on the EAA-induced responses in the striatum.

Possible mechanism of rapid eye movement sleep deprivation induced increase in Na-K ATPase activity

Rapid eye movement sleep deprivation increases Na-K ATPase activity and decreases aminergic neuronal firing rate as well as norepinephrine degrading enzyme, monoamine oxidase, activity. On the other hand, norepinephrine is known to increase Na-K ATPase activity. Hence, this study was conducted to find if the deprivation induced increase in Na-K ATPase activity is mediated by norepinephrine. Rapid eye movement sleep deprived rats were injected with either alpha-1 or beta adrenoceptor antagonist or alpha-2 adrenoceptor agonist and after 8 h the Na-K ATPase activity of the brain was estimated. In an attempt to simulate in vivo conditions, norepinephrine was added to an in vitro brain homogenate preparation in the presence or absence of alpha or beta adrenoceptor blockers and the enzyme activity was estimated. The results showed that the enzyme activity was decreased by alpha-1 antagonist as well as by alpha-2 agonist treatment in in vivo preparations. Norepinephrine increased enzyme activity in the in vitro preparation and the increase was prevented by the alpha-1 antagonist. The results of this study suggest that rapid eye movement sleep deprivation induced increase in Na-K ATPase activity may be mediated by norepinephrine acting on either alpha-1 and/or alpha-2 receptors.

Activation of alpha-adrenoceptors indirectly facilitates sodium pumping in frog motoneurons

The effects of clonidine on Na+ pumping in motoneurons of the isolated frog spinal cord was investigated using sucrose gap recordings from ventral roots. Na+ pump activity, induced in motoneurons either by tetanizing the dorsal root or by rapidly exposing the cord to normal medium following 30 min in K(+)-free Ringer's solution (K(+)-activated hyperpolarizations), was increased by application of clonidine (100 microM). These actions of clonidine were blocked by the preferential alpha 2-adrenergic antagonist yohimbine, but not by alpha 1-adrenergic antagonist prazosin or the beta-blocker propranolol. Clonidine's effects on Na+ pumping appeared to be indirect (presumably via interneurons) because its effects on K(+)-activated hyperpolarizations were reduced by tetrodotoxin (TTX) or high concentrations of Mg2+. This indirect mechanism involved activation of non-NMDA excitatory amino acid receptors. Thus, in the presence of clonidine, CNQX, but not APH, limited the ability of clonidine to enhance K(+)-activated hyperpolarizations. The AMPA receptor may play a role in the process, K(+)-activated hyperpolarizations were augmented by the presence of AMPA; NMDA had no effect. The present results are consistent with the idea that activation of alpha 2-adrenoceptors produces the following: the release of excitatory amino acids from interneurons; the activation of non-NMDA receptors on motoneurons; increased Na+ influx and loading and increased Na+ pump activity.

The role of noradrenaline in the differentiation of amphibian embryonic neurons

The possibility that monoamines might act as signalling molecules during the early development of the nervous system has been examined in embryos of the amphibian Xenopus laevis. The distributions of 5-hydroxytryptamine, dopamine, noradrenaline and their precursor, dopa, were determined from the fertilized egg up to the late neurula stages using High Performance Liquid Chromatography, formaldehyde-induced fluorescence and antibody staining. 5-hydroxytryptamine was not detected until the tail bud stage. The fertilized egg contained significant concentrations of dopa (10(−6) M) and dopamine (10(−7) M). Both monoamines persisted with little change in concentration up to the late neurula stage. Early neurula stage embryos contained very low levels of noradrenaline. Aldehyde-induced fluorescence showed that monoamines are localized in dorsal regions of the embryo, in ectoderm and mesoderm cells. Monoamines were not present in endoderm cells. Immunocytochemical staining showed dopamine predominantly in the ectoderm, except in future neural regions where it was found also in the mesoderm. Dopamine staining was always most intense in dorsal regions of the embryo. The consequences for subsequent neuronal differentiation of interfering with the biosynthesis and receptor binding of monoamines during neurulation was assayed. Neuronal differentiation was monitored quantitatively in cultures set up as the neural tube closed and qualitatively in intact tadpoles that were left to develop for two days after washout of test reagent. The number of neurons, the number of muscle cells and the total number of differentiated cells were counted after 18–24 hours of culture. Comparison of the number of neurons that differentiated from control and treated embryos showed that inhibition of dopamine beta-hydroxylase, the enzyme catalysing the conversion of dopamine to noradrenaline, during the neural plate stages reduced substantially subsequent neuronal differentiation. The differentiation of myocytes and the total number of differentiated cells were not affected. Exogenous noradrenaline (10(−6) M) or dopamine (10(−6) M) could increase the number of neurons that differentiated subsequently in culture. Interfering with noradrenaline binding to receptors with receptor antagonists during neurulation showed that alpha-adrenergic receptor antagonists reduced substantially the subsequent differentiation of neurons. The differentiation of myocytes and the total number of differentiated cells were not affected. The effect of alpha-adrenergic receptor antagonists was overcome by the simultaneous inclusion of noradrenaline or alpha-receptor agonists, but not agonists at beta-adrenergic receptors. The quantitative reduction in the differentiation of neurons was paralleled by defects in the Central Nervous System of intact tadpoles.(ABSTRACT TRUNCATED AT 400 WORDS)

Na+,K(+)-ATPase and acetylcholinesterase activities: changes in postnatally developing rat brain induced by bilirubin

Na+,K(+)-ATPase, Mg(++)-ATPase, and acetylcholinesterase activities were determined in brain homogenates of rats in different ages, which were decapitated 30 min after administration of various bilirubin doses. Bilirubin serum and brain tissue levels should be dependent upon the dose administered. At these concentrations, a progressive enzyme inactivation was observed, which reached 25-30% for acetylcholinesterase and 70-80% for Na+,K(+)-ATPase in neonate rats and 15-20% for acetylcholinesterase and only 30-40% for Na+,K(+)-ATPase in the brain of aged rats (20 mo). However, Mg(++)-ATPase activity was not affected by bilirubin deposition in the developing brain. Moreover, brain albumin content increased 53% in suckling, 40% in adult, and 33% in aged rats at high drug administration. These results may indicate an opening of the blood-brain barrier and a bilirubin entry into the rat brain. The bilirubin immediate toxic effects on brain acetylcholinesterase and Na+,K(+)-ATPase, and probably on brain electrical activity, may be modulated by the developmental state of membrane-bound enzymes.

Cyclic AMP mediates inhibition of the Na(+)-K+ electrogenic pump by serotonin in tactile sensory neurones of the leech

1. Serotonin (5-HT) reduced the after-hyperpolarization (AHP) amplitude in tactile sensory neurones (T) but not in pressor (P) or nociceptive (N) cells of the leech. 2. Adenylate cyclase activators, phosphodiesterase inhibitors and membrane permeant analogues of cyclic adenosine monophosphate (cyclic AMP) mimicked the effect of 5-HT in reducing the AHP amplitude in T neurones. 3. Ionophoretic injection of cyclic AMP in T cells reduced the AHP amplitude, while cyclic guanosine monophosphate (cyclic GMP) or adenosine-5'-monophosphate (AMP) were without effect. 4. Inhibition of adenylate cyclase by the drug RMI 12330A (also known as MDL 12330A) suggested that 5-HT reduced the AHP amplitude through cyclic AMP. 5. 8-Bromoadenosine-3'-5'-cyclic monophosphate (8-Br-cyclic AMP) was still able to reduce the AHP amplitude after blocking the Ca(2+)-activated K+ conductance with CdCl2 and converted the normal hyperpolarization which follows the intracellular injection of Na+ into a depolarization. In addition, the cyclic AMP analogue slowed down and reduced the repolarization usually induced by CsCl after perfusion with K(+)-free solution. It is proposed that, in T sensory neurones, cyclic AMP mediates the inhibition of the Na(+)-K+ electrogenic pump induced by 5-HT application.

Time changes in Na+,K(+)-ATPase, Mg(++)-ATPase, and acetylcholinesterase activities in the rat cerebrum and cerebellum caused by stress

Na+,K(+)-ATPase, Mg(++)-ATPase, and acetylcholinesterase activities were determined in homogenated rat cerebrum and cerebellum in unstressed animals (control) and exposed to cold and immobilization for 45-180 min. Na+,K(+)-ATPase and Mg(++)-ATPase activities were not affected within the first 80 min of stress, while they were increased about 50-70% after 120-180 min, where the maximum enzyme stimulation was observed. However, acetylcholinesterase activity was increased considerably by less than 45 min of stress and reached a plateau in 80-180 min to a higher value in the cerebrum (approximately equal to 100%) than in the cerebellum (approximately equal to 40%) related to the control. Our results suggest that: a) The stress used can stimulate acetylcholinesterase in a different way and more quickly than Na+,K(+)-ATPase and Mg(++)-ATPase; b) acetylcholinesterase in the cerebellum is stimulated to a lower level than in the cerebrum by stress, probably because of the presence of a relatively small cerebellar cholinergic innervation.

Brain Na+/K(+)-ATPase regulation by serotonin and norepinephrine in normal and kindled rats

In this work we confirmed the activation of rat brain Na+/K(+)-ATPase by norepinephrine (NE) and observed a variable response of the enzyme according to the brain region considered. In isolated neuronal or glial fractions from normal cerebral cortices, we studied the response of the enzyme to increasing concentrations of serotonin (5-HT) (10(-9)-10(-3) M). A dose-dependent response over basal values was present in glial fractions, beginning at 10(-6) M. No such response was obtained in the neuronal fractions. In amygdaloid kindled brains, the pattern of activation by NE was different than in controls: less pronounced (cortex, brainstem, and diencephalon), inhibition-activation (cerebellum), or no change (striatum). The activation of Na+/K(+)-ATPase by 5-HT observed in the control glial fraction was not present in the kindled glial fraction. In conclusion, 5-HT seems to activate Na+/K(+)-ATPase preferentially in glial cells, and the kindling process markedly modifies this regulation. The normal response to NE in brain homogenates is less altered by kindling than is the response to 5-HT in the same regions.

Effect of norepinephrine and heart rate on intracellular sodium activity and membrane potential in beating guinea pig ventricular muscle

The effect of 3 microM norepinephrine (NE) on intracellular sodium activity (aiNa) and resting membrane potential was studied by continuous intracellular recordings with a conventional and an ion-selective microelectrode. The electrodes were impaled simultaneously in small (diameter, 0.3 mm) superfused trabeculae of the beating guinea pig ventricle at 37 degrees C. In the absence of NE, changes of the beating rate produced an increase of aiNa by 1.5 +/- 0.17 mM (from 0 to 1 Hz) and 1.9 +/- 0.47 mM (from 0 to 2 Hz). In the presence of NE, there was a very small significant increase of aiNa during constant stimulation (1 Hz) and at at [K+]o of 4.7 and 11.5 mM. After 7 minutes of exposure, aiNa increased by 0.5 +/- 0.19 mM (mean +/- SEM, n = 4) at [K+]o of 4.7 mM and by 0.5 +/- 0.22 (n = 6) at [K+]o of 11.5 mM. Resting membrane potential became more positive by 1 mV at both levels of [K+]o. The effect of NE became also clearly manifest from the configurational changes of action potentials (profound increase in plateau height and duration). Stimulation of the Na(+)-K+ pump by NE became manifest from the changes of resting membrane potential and aiNa after abrupt cessation of stimulation. The magnitude and the rate of the decrease in aiNa and the initial rate of hyperpolarization were significantly greater in the presence of NE than in its absence.(ABSTRACT TRUNCATED AT 250 WORDS)

Electrophysiologic effects of isoproterenol in patients with atrioventricular reentrant tachycardia treated with flecainide

The electrophysiologic effects of isoproterenol in patients treated with flecainide for atrioventricular (AV) reentrant tachycardia were studied to evaluate the mechanism of tachycardia inducibility after isoproterenol and the value of isoproterenol challenge as a predictor of spontaneous arrhythmia recurrence. Seventeen patients underwent electrophysiologic study before and after oral flecainide administration and after the addition of isoproterenol to flecainide. No patient had inducible sustained supraventricular tachycardia after flecainide alone. Two patients had inducible sustained and six had inducible nonsustained tachycardia after isoproterenol was added to flecainide. The patients were then followed up on the same flecainide dose they received at the time of the electrophysiologic study.

FINDINGS

1) Flecainide treatment prolonged HV and VA intervals, and the addition of isoproterenol did not affect these variables. 2) Isoproterenol shortened anterograde and retrograde block cycle length and the refractory period of the accessory pathway and the AV node. It also decreased the tachycardia cycle length, an effect that was due solely to shortening of AV node conduction time. 3) Flecainide treatment prevented tachycardia induction by affecting retrograde conduction over the accessory pathway. Isoproterenol allowed for tachycardia induction and for more sustained episodes of tachycardia by reversing the effect of flecainide on retrograde accessory pathway conduction. 4) Tachycardia recurred during follow-up in all three patients in whom tachycardia of greater than or equal to 10 s duration was induced after isoproterenol but in no patient who had no or shorter episodes of induced tachycardia (and who did not have a change in medical regimen).

CONCLUSIONS

1) Isoproterenol reverses flecainide-induced prolongation of block cycle length and refractory periods of the accessory pathway and AV node. 2) Isoproterenol reverses flecainide-induced prevention of tachycardia induction through reversal of the effects of flecainide on the retrograde accessory pathway. 3) The addition of isoproterenol during flecainide restudy is valuable in predicting long-term drug efficacy.

Inhibition by dopamine of (Na(+)+K+)ATPase activity in neostriatal neurons through D1 and D2 dopamine receptor synergism

The (Na(+)+K+)ATPase, an integral membrane protein located in virtually all animal cells, couples the hydrolysis of ATP to the countertransport of Na+ and K+ ions across the plasma membrane. In neurons, a large portion of cellular energy is expended by this enzyme to maintain the ionic gradients that underlie resting and action potentials. Although neurotransmitter regulation of the enzyme in brain has been reported, such regulation has been characterized either as a nonspecific phenomenon or as an indirect effect of neurotransmitter-induced changes in ionic gradients. We report here that the neurotransmitter dopamine, through a synergistic effect on D1 and D2 receptors, inhibits the (Na(+)+K+)ATPase activity of isolated striatal neurons. Our data provide unequivocal evidence for regulation by a neurotransmitter of a neuronal ion pump. They also demonstrate that synergism between D1 and D2 receptors, which underlies many of the electrophysical and behavioural effects of dopamine in the mammalian brain, can occur on the same neuron. In addition, the results support the possibility that dopamine and other neurotransmitters can regulate neuronal excitability through the novel mechanism of pump inhibition.

The effect of amiloride on the cardiac chronotropic responses to isoproterenol in myocardial aggregate cells in culture

The purpose of this study was to test the hypothesis that amiloride alters the response of cardiac myocytes to isoproterenol. Myocardial cell aggregates were prepared from 7 day-old chick embryos maintained in culture for 72 hrs before study. Isoproterenol, 10-8 M to 10-5 M, significantly (P less than 0.05) increased contractile frequency of myocardial aggregates. The effects of isoproterenol were maximum within 5 min. of exposure and declined thereafter. In the absence of isoproterenol, amiloride, at 10-6 M and 10-7 M produced a transient decrease in contractile frequency while amiloride at 10-5 M produced a significant (P less than 0.05) decrease in contractile frequency. Amiloride significantly (P less than 0.05) increased the effect of isoproterenol on cardiac contractile frequency. There was a greater and more sustained response to isoproterenol in the presence of amiloride. Furthermore, the magnitude of these effects were greater with higher concentrations of amiloride. These data indicate that amiloride accentuates the cardiac chronotropic response to isoproterenol and suggest that, because amiloride inhibits sodium entry in these cells, change in intracellular sodium may be one of the mechanisms mediating the chronotropic action of isoproterenol on the heart.

A prolonged post-tetanic hyperpolarization in rat hippocampal pyramidal cells in vitro

The post-tetanic sequelae of trains of synaptic stimuli (50 pulses at 5 or 10 Hz) were studied with intracellular recordings from rat hippocampal neurons in vitro. In a large proportion of CA1 neurons, stimulation of afferent fibers was followed by a prolonged membrane hyperpolarization (peak amplitude approximately 6 mV) that was associated with a decrease in neuronal input resistance (approximately 33%) that lasted from tens of seconds to over 1 min. Antidromic stimulation or activation of cells with intracellular current injection did not elicit this post-tetanic hyperpolarization (PTH). The PTH could be elicited in chloride (Cl-)-loaded cells, its null potential shifted in response to changes in extracellular potassium ([K+]o), and it was significantly reduced by 5-10 mM extracellular cesium (Cs+). The K(+)-dependent PTH may also be calcium (Ca2+) dependent as its amplitude and associated conductance increase were sensitive to changes in [Ca2+]o. The PTH was enhanced by treatments that increase Ca2+ entry into cells including perfusion with elevated [Ca2+]o, with picrotoxin or with tetraethylammonium ion (TEA). The K+ conductance blocker 4-AP had no consistent effect on the PTH. The PTH was potently blocked by the membrane-permeant forms of cAMP, dibutyryl- and 8-bromo-cAMP. However, phorbol esters that activate protein kinase C and carbachol, which usually block the same potential that is blocked by cAMP, did not depress the PTH. The cardiac glycosides dihydro-ouabain and strophanthidin had only small and variable effects on the PTH.(ABSTRACT TRUNCATED AT 250 WORDS)

Lithium distribution in mania: single-dose pharmacokinetics and sympathoadrenal function

We examined lithium distribution after a single dose of 25 mEq in 14 drug-free manic patients. Lithium concentrations were measured in plasma, red blood cells, and urine. Maximum concentrations of lithium, times at which they were attained, and influx and efflux rate constants for extracellular fluid, red blood cell, and muscle-like compartments were estimated using a three-compartment pharmacokinetic model. Tissue lithium concentrations may continue to increase for hours after plasma lithium concentrations have peaked. Rate constants for absorption, excretion, and influx and efflux for the tissue compartments were similar to those previously reported for normal subjects. Rate constants for transport into and out of the tissue compartments correlated negatively with norepinephrine or epinephrine excretion and positively with the plasma/red cell Na+ gradient. Rate constants for efflux from red blood cell and muscle compartments correlated with measures of adrenocortical function and were higher in dexamethasone nonsuppressors than in suppressors. These data show that distribution of lithium may be related to sympathodrenal activity and Na+ distribution in manic patients.

The effect of lithium on cation transport measured in vivo in patients suffering from bipolar affective illness

We have investigated cation transport in vivo in patients being treated with lithium for bipolar affective illness by studying the disposition of rubidium after an oral load of rubidium chloride. The rate of erythrocyte cation transport was increased in the patients when compared with matched healthy volunteers. However, the rate of in-vivo erythrocyte rubidium accumulation in the euthymic treated patients was significantly lower than in a matched group of unmedicated manic patients. The regulation of specific pathways for cation transport may be altered in individuals predisposed to affective illness.

The measurement of transmembrane cation transport in vivo in acute manic illness

We have used a novel technique to assess the transport of cations across the erythrocyte membrane in vivo in unmedicated patients suffering an acute manic illness. The results show that erythrocyte cation transport via the sodium-pump enzyme Na+,K+-ATPase is increased in manic patients compared with healthy controls.

Changes of the membrane potential in striatal synaptoneurosome, synaptosome and membrane sac preparations induced by glutamate, kainate and aspartate as measured with a cyanine dye DiS-C2-(5)

The effects of glutamate, kainate and aspartate on the membrane potential of striatal synaptoneurosome, synaptosome and membrane sac preparations were studied by using a potential sensitive cyanine dye DiS-C2-(5). Excitatory amino acids glutamate and aspartate had a depolarizing effect on synaptoneurosomes. 7.9 microM glutamate and 2.8 microM aspartate produced a half-maximal response. Depolarizations induced by glutamate and aspartate were dependent on the concentration of extracellular sodium ions, a maximal response occurred at around 40 mM of external Na+. Kainate induced a dual effect on synaptoneurosomes. In a standard Na+-based medium a hyperpolarization, likely due to inhibition of a presynaptic sodium-dependent glutamate uptake, predominated over a postsynaptic kainate receptor-mediated depolarization that was observed when electrogenic glutamate uptake was inhibited. This interpretation was supported by results obtained with synaptosome and membrane sac preparations. In a standard Na+-based medium kainate had a hyperpolarizing effect on synaptosomes while in the membrane sac preparation kainate induced a depolarization.

Isoproterenol, DBcAMP, and forskolin inhibit cardiac sodium current

We studied the effects of isoproterenol (ISP), dibutyryl adenosine 3',5'-cyclic monophosphate (DBcAMP), and forskolin on the sodium current (INa) of guinea pig ventricular myocytes using the tight-seal, whole cell voltage-clamp method. The extracellular [Na+] [( Na+]o) was decreased to 60 mM by replacing NaCl with sucrose (temperature, 32-33 degrees C). Ionic currents other than Na+ were suppressed using appropriate channel blockers. Depolarizing clamp pulse (duration, 30 ms) was applied at a rate of 0.2 Hz from a holding potential of -80 mV. ISP (1 microM) decreased the peak INa by 34% from 6.1 +/- 1.9 (SD) nA (control) to 4.0 +/- 1.5 nA (n = 7). The inhibition was more prominent at less negative potentials and disappeared in the presence of a beta-blocker (10 microM atenolol). The effects of DBcAMP (1-5 mM) and forskolin (3 microM) mimicked those of ISP and depressed the peak INa reversibly. DBcAMP (5 mM) shifted the inactivation curve of INa [h infinity-membrane potential (Em) relationship] to a hyperpolarizing direction, by 3.4 +/- 0.8 mV (n = 5). These findings suggest that ISP inhibits the cardiac INa+, probably by altering the gating mechanism of the Na+ channel, and that the effect is secondary to the increased levels of intracellular cAMP, with possible acceleration of cAMP-dependent phosphorylation of the channel.

Subacute noradrenergic agonist infusions in vivo increase Na+, K+-ATPase and ouabain binding in rat cerebral cortex

In order to investigate the specificity of noradrenergic effects on Na+, K+-ATPase, we infused noradrenergic agonists into the cerebral ventricles of rats, with or without depletion of forebrain norepinephrine. Infusion of norepinephrine, isoproterenol, or phenylephrine increased ouabain binding in intact rats, whereas clonidine infusion decreased binding. Depletion of forebrain norepinephrine by destruction of the dorsal noradrenergic bundle reduced ouabain binding. Norepinephrine infusion reversed the effect of dorsal bundle lesion; isoproterenol and phenylephrine increased ouabain binding in lesioned rats, but did not restore the effect of the lesions. Clonidine had no effect in lesioned rats. Effects on Na+, K+-ATPase activity were similar, but smaller. These results suggest that stimulation of both alpha 1- and beta-noradrenergic receptors may be necessary for optimal Na+, K+-ATPase, and that clonidine reduces Na+, K+-ATPase indirectly through decreased norepinephrine release.

Acute calcium entry blockade inhibits the blood pressure but not the hormonal responses to angiotensin II

The effects of acute calcium entry blockade by isradipine (IS) and placebo (P) on the haemodynamic and humoral responses to angiotensin II (A II) have been compared in two groups of 9 patients with essential hypertension. During 4 sequential periods each of 20 min, an i.v. infusion of A II 0, 2, 4 and 8 ng.kg-1.min-1 was given before (control) and 30 min after the oral administration either of IS or P. After IS, both the blood pressure and the angiotensin II-induced pressor effect were significantly reduced. Isradipine increased the heart rate and this cardio-acceleration was potentiated by A II. In contrast, when A II was infused in the absence of IS, heart rate tended to decrease. IS stimulated plasma renin activity and reduced plasma aldosterone. However, it did not affect either the inhibition of plasma renin activity or the rise in plasma aldosterone in response to A II. In conclusion, acute calcium entry blockade in patients with essential hypertension reduces the pressor response to A II, but not the A II-induced inhibition of renin and increase in plasma aldosterone.

Noradrenaline antagonizes and ouabain potentiates the effects of N-methyl-D-aspartate on rat cerebellar cyclic GMP production

Noradrenaline potently antagonizes the effects of N-methyl-D-aspartate (NMDA) (80 microM) on cyclic GMP production in immature rat cerebellar slices in vitro (IC50 = 0.6 microM). The effect is stereospecific (D-noradrenaline, IC50 = 100 microM), and also observed with adrenaline (IC50 = 0.5 microM) and isoprenaline (IC50 = 1.2 microM). The alpha 1-adrenoceptor agonists methoxamine or phenylephrine or the mixed alpha 1/alpha 2 agonists oxymetazoline or xylometazoline (100 microM) do not block the effects of NMDA, but the alpha 2-adrenoceptor agonist clonidine is weakly active (IC50 = 200 microM). Salbutamol and terbutaline were also inactive except at high concentrations (300 microM), as were a number of other catechol and phenylethylamine derivatives. The antagonistic effects of noradrenaline on the NMDA response were insensitive to phentolamine, atenolol, or propranolol (up to 100 microM), but were blocked by the alpha 2 antagonist idazoxan (1-10 microM). The Na+,K+-ATPase inhibitor ouabain (0.1-10 microM) markedly potentiates the effects of NMDA in this model, and also antagonizes and reverses the ability of noradrenaline (10 microM) to block the effects of NMDA. The results suggest that noradrenaline and Na+,K+-ATPase activity have potent modulatory effects on the NMDA response.

Role of ion conductance changes and of the sodium-pump in adrenaline-induced hyperpolarization of rat diaphragm muscle fibres

The ionic mechanism of membrane hyperpolarization induced by adrenaline in rat diaphragm muscle fibres was studied. Removal of the extracellular K+ ([K+]o) from Krebs-Ringer solution initially increased the resting membrane potential and then caused an increase in the intracellular Na+ activity ([Na+]i) and a decrease in the intracellular K+ activity ([K+]i). All the changes were maintained for more than 3 h. Application of ouabain (0.1 mM) or lowering the temperature rapidly reduced the resting potential by about 10 mV in the K+-free solution. It then produced further progressive decreases in resting potential and in [K+]i and a progressive increase in [Na+]i. These observations indicate that an electrogenic Na-pump operates in the K+-free solution. Removal of most of the Cl- in the K+-free solution did not affect the resting potential or the magnitude of the initial decrease produced by ouabain, despite an increased input resistance; this result implies a passive distribution of Cl-. Adrenaline (30-60 microM) either added to the bathing solution or applied to the membrane by ionophoresis produced a hyperpolarization (3-10 mV: adrenaline hyperpolarization), the amplitude of which was decreased with a rise in [K+]o and increased with a reduction in [K+]o, but unaffected by the removal of Cl-. Adrenaline produced an increase in input resistance, the relative magnitude (17-18%) of which was constant whether external K+ or Cl- was removed. In contrast, a conditioning membrane hyperpolarization hardly affected the resistance. Ouabain (0.1 mM) or low temperature (8-10 degrees C) abolished both the hyperpolarization and the increased input resistance induced by adrenaline. The [K+]i, [Na+]i and the peak of the action potential remained unchanged after a 20 min exposure to adrenaline (30 microM). The hyperpolarization induced by the replacement of all Na+ with Tris (Tris-hyperpolarization) in the K+-free solution was depressed by 39% during the early period (4-31 min) of exposure to adrenaline (30 microM), while it was enhanced by 26% during the later period (80-130 min). The initial depression suggested a decrease in the ratio of the membrane permeability for Na+ (PNa) to that for K+ (PK). These results suggest that the adrenaline hyperpolarization is generated largely by a decrease in PNa/PK, which is associated with the activity of the Na-pump.

Adrenaline-induced K+ efflux results in sodium pump stimulation in a sympathetic ganglion

The potassium-activated hyperpolarization (KH) was used as an index of electrogenic Na+ pumping in bullfrog sympathetic ganglia. This response was evoked by storing ganglia in K-free Ringer's solution and briefly introducing normal Ringer's solution containing 2 mM K+ at regular intervals. The apparent EC50 for K+ was 2.21 mM (range 0.88-3.54 mM, for n = 5) and at least 10 mM K+ was required to produce a maximal KH response. Adrenaline, which produces membrane hyperpolarization by increasing K+ conductance (gK), increased the amplitude of KH responses. When the K+ efflux accompanying the adrenaline-induced hyperpolarization (AdH) was blocked with 2 mM Ba2+, the KH was no longer potentiated. It is suggested that the K+ moving out of the cells during the AdH accumulates extracellularly and stimulates the Na+ pump.

Brain Na+,K+-ATPase: alteration of ligand affinities and conformation by chronic ethanol and noradrenergic stimulation in vivo

These experiments examined effects of chronic ethanol, repeated noradrenergic stimulation or inhibition, and ethanol combined with the noradrenergic treatments on regulation of Na+,K+-ATPase. Chronic treatment with ethanol reduced the sensitivity of K+-p-nitrophenyl-phosphatase to ethanol, increased affinity for K+, reduced the sensitivity of K+ affinity to ATP or ethanol, and reduced delta H and delta S for K+ activation and for the E1-E2 transition. These effects were all opposite to those of ethanol added in vitro. Treatment with yohimbine had the opposite effects on ethanol sensitivity, K+ affinity, K+ interactions with ethanol and ATP, and thermodynamic parameters for cation activation or conformational change. These effects were similar to those of norepinephrine in vitro. The effects of yohimbine treatment were eliminated or reduced in rats also treated with ethanol. Depletion of norepinephrine had effects opposite to those of yohimbine. These data are consistent with a reduction in membrane fluidity, at least in the vicinity of Na+,K+-ATPase, during ethanol tolerance. Exposure to norepinephrine, in vitro or in vivo, had effects on Na+,K+-ATPase that were similar to those of increased membrane fluidity.

(Na+,K+)-ATPase and noradrenergic regulation: effects of cardiac glycoside treatment and noradrenergic manipulations

We examined effects of treatment with cardiac glycosides, in combination with noradrenergic stimulation or depletion, on (Na+,K+)-ATPase activity in rat cerebral cortex, heart, and kidney. Treatment with digitoxin increased the apparent number of (Na+,K+)-ATPase sites in heart, cerebral cortex, and kidney. Ouabain, which crosses the blood-brain barrier poorly, did not affect enzyme in brain but was otherwise similar. Norepinephrine depletion prevented the increase in heart but not in cerebral cortex. Noradrenergic stimulation increased the number of sites in cerebral cortex and in heart. In rats treated with digitoxin, noradrenergic stimulation increased enzyme activity further in heart but not in cerebral cortex. Examination of effects on noradrenergic receptor binding and on norepinephrine metabolite concentrations suggested that, while in heart cardiac glycosides appeared to increase norepinephrine release, in brain there was no effect on release but there may have been appreciable inhibition of norepinephrine reuptake under stimulated conditions.

Beta-adrenergic modulation of the Na+-K+ pump in frog skeletal muscles

Adrenaline markedly increased the ouabain-sensitive 22Na+-efflux by stimulating the Na+-K+ pump in frog skeletal muscle. The facilitatory effects of adrenaline had the following properties. The effects of adrenaline on the ouabain-sensitive Na+-efflux were observed at concentrations greater than 0.1 microM and the magnitude increased with concentration up to 10 microM. At a concentration of 30 microM, adrenaline markedly augmented the ouabain-sensitive Na+-efflux, but other biogenic amines were less effective (noradrenaline and dopamine) or ineffective (histamine and serotonin). The increase of Na+-efflux induced by 1 microM adrenaline was blocked by 3 microM propranolol, but not by 3 microM phenoxybenzamine. The properties of the facilitatory action of adrenaline on the ouabain-sensitive Na+-efflux suggest that beta-adrenoceptors have an important role in modulating the Na+-K+ pump activity in the skeletal muscle membrane. The protein complex localized in excitable membranes, namely the Na+-K+ ATPase-beta-adrenoceptor complex, may be the functional unit which operates the membrane machinery driving the Na+-K+ pump.

Serotonin and Retzius cell depress the hyperpolarization following impulses of leech touch cell

Intracellular recordings from T mechanosensory cells of Hirudo medicinalis showed, as previously demonstrated, that repetitive firing is followed by a long-lasting hyperpolarization. Serotonin application at two concentrations (1 microM and 50 microM) depressed this hyperpolarization by up to 2/3; the effect was dose-dependent, long-lasting and reversible. Intracellular stimulation of giant serotonergic neurons (Retzius cells, Rz) mimicked serotonin perfusion: the effect was proportional to the number of spikes fired by Retzius cells. The combined use of intracellular iontophoretic injection of horseradish peroxidase and lucifer yellow indicated the possible sites of contact between Rz and T cells. The effect of serotonin, released by Rz cells, is discussed with respect to its possible physiological significance.

Examination of the role of the electrogenic sodium pump in the adrenaline-induced hyperpolarization of amphibian neurones

The effect of adrenaline and acetylcholine (ACh) on the membrane potential of Rana pipiens sympathetic ganglia was examined by means of the sucrose gap recording technique. Adrenaline (1-50 microM) consistently produced a hyperpolarization (Adrh) which was not reduced by Ringer solution containing 10 mM-Mn2+, nor by Ringer solution where the Na+ concentration was reduced from 100 to 30 mM. High doses of ACh (10 mM) produced a biphasic response, a depolarization (AChd) followed by an after-hyperpolarization (ACha.h.p.). Ringer solution containing 100 mM-Li+ (rather than 100 mM-Na+) or 10 microM-ouabain blocked the ACha.h.p. and reduced the Adrh. The AChd was essentially unchanged. Ringer solution containing 0.2 mM-K+ (rather than 2 mM-K+) blocked part of the ACha.h.p. whereas the Adrh was enhanced. Ringer solution containing 6 mM-K+ reduced the amplitude of the Adrh. The Adrh and the antidromically evoked action potential after-hyperpolarization (antidromic a.h.p.) reversed polarity at approximately the same membrane potential. These data do not support the hypothesis that the Adrh results from activation of the electrogenic sodium pump. It is tempting to speculate that the response may be generated by an increase in potassium conductance (gK) which is especially sensitive to manipulations which result in sodium pump inhibition.